Heterothallic mating type switching in Saccharomyces cerevisiae is RAD52 dependent

The effects of RAD52 epistasis group genes on various types of spontaneous mitotic recombination in Saccharomyces cerevisiae

The role of RAD52 epistasis group genes on spontaneous mitotic recombination was examined using three different types of spontaneous mitotic recombination in Saccharomyces cerevisiae. The spontaneous recombination between homologous sequences in a plasmid and a chromosome was essentially unaffected by null mutations in any of the RAD52 epistasis group genes. Recombination between genes in separate autonomously replicating plasmids was reduced 833-fold in a rad52 null mutant, but only 2- to at most 20-fold in rad50, 51, 54, 55, 57 null mutants. Recombination between tandemly repeated heteroalleles in an autonomously replicating plasmid was reduced almost 100-fold in a rad52 null mutant, but is either unaffected or slightly increased in rad50, 51, 54, 55, 57 null mutants. The finding that RAD50, 51, 54, 55, 57 are dispensable or marginally involved in these spontaneous recombinations suggests further that spontaneous mitotic recombination in S. cerevisiae might be processed by other than RAD52 epistasis group.

DNA damage induced mating type switching in Saccharomyces cerevisiae

Haploid cells of the yeast Saccharomyces cerevisiae are able to undergo a differentiation-like process: they can switch their mating type between the a and the alpha state. The molecular mechanism of this interconversion of mating types is intrachromosomal gene conversion. It has been shown in a variety of studies that mating type switching in heterothallic strains can be induced by DNA damaging agents, and that different DNA damaging agents differ in the length of incubation after treatment required for induction. Because X-rays induce switching immediately after irradiation and because the DNA double-strand break repair pathway is required for switching, the event initiating heterothallic mating type switching is likely to be a DNA double-strand break. Therefore the assay for heterothallic mating type switching may screen for the induction of DNA double-strand breaks. Several aspects indicating a relationship of mating type switching to mechanisms associated with carcinogenesis are discussed.

Interchromosomal and intrachromosomal recombination in rad 18 mutants of Saccharomyces cerevisiae

The frequency of intra- and interchromosomal recombination was determined in RAD18 and rad18 deletion and rad18-3 mutant strains. It was found that spontaneous interchromosomal recombination at trp5, his1, ade2, and MAT was elevated 10- to 70-fold in the rad18-3 and rad18 delta mutants as compared to the RAD+ strains. On the other hand the frequencies of spontaneous intrachromosomal recombination for the his3 delta 3', his3 delta 5' and the his4C-, his4A- duplications and for heterothallic mating type switching were only marginally elevated in the rad18 deletion mutant, and recombination between ribosomal DNA repeats was only 2-fold elevated in the rad18-3 mutant. These differences may be due to a haploid versus diploid specific difference. However interchromosomal recombination was elevated 40-fold and intrachromosomal recombination was only marginally (1.5-fold) elevated in a diploid homozygous for rad18 delta, arguing against a haploid versus diploid specific difference. Possible explanations for the difference in the elevated levels of intra- versus interchromosomal spontaneous recombination are discussed.

DNA-damaging agents show different kinetics in induction of heterothallic mating-type switching during growth after treatment in yeast

Heterothallic mating-type switching in Saccharomyces cerevisiae has been shown to be inducible by DNA-damaging agents (Schiestl and Wintersberger, 1983). Different DNA-damaging agents differ greatly in their kinetics of induction during incubation after treatment. Irradiation with X-rays resulted in an increase in the frequency immediately after exposure and no further increase was seen during incubation after treatment. Nitrous acid and 4-nitroquinoline-N-oxide, on the other hand, did not show any increase in frequency immediately after treatment, but require post-treatment incubation to produce an increased frequency of heterothallic mating-type switching. UV irradiation and ethyl methanesulfonate result in induction to certain levels immediately after treatment, but further induction was seen during post-treatment incubation. The results may indicate that certain kinds of DNA damage require repair of replication to be converted into recombinogenic lesions.